1. Field of the Invention
This invention relates to improvements in seismic energy sources used to create pulses in a liquid medium such as water. In particular, it relates to improvements in recoil reducing systems of gas guns which create pulses in the medium by introducing therein gas under high pressure.
2. Description of the Prior Art
In marine seismic prospecting it has been common to use seismic sources known as gas guns to induce pulses of desired characteristics in the liquid medium. One frequently used kind of gas gun is shown in U.S. Pat. No. 4,193,472 (1980) to Kirby. This kind of gas gun comprises a barrel having an outlet at its lower end. The space inside the barrel defines an explosion chamber. Explosive gas is introduced into the explosion chamber and then ignited. A reaction plate is attached to the barrel and disposed below the outlet, so that gas from the explosion impinges upon the reaction plate before escaping into the surrounding medium. Thus the force exerted by the escaping gas on the barrel is at least partially balanced by the force exerted by the escaping gas upon the reaction plate. Recoil of the barrel is thereby reduced. The reaction plate is usually attached to the barrel by means of sturdy metal rods which can withstand the enormous force exerted by gas from the explosion upon the plate. These rods are usually welded to the reaction plate to form sturdy connections that can withstand the explosion.
When the gas escaping from the outlet impinges upon the reaction plate, the tremendous force exerted on the plate causes the portion of the plate not directly supported by the metal rods to flex downwardly with respect to the directly supported portion. Since the supporting metal rods are welded rigidly onto the reaction plate the metal rods are also caused to flex. It has been found that after a large number of explosions in the gun barrel, the constant flexing causes metal fatigue of the support rods so that these rods break just above the reaction plate. It is thus desirable to reduce the flexing of the support rods so that the life of the support rods is extended.
Rubber springs have been used in conjunction with gas guns in the seismic art. In U.S. Pat. No. 4,043,420 (1977) Zens et al discloses a gas detonator to be used in seismic surveys. The detonator comprises a circular bottom plate coupled to the earth, an annular top plate over the bottom plate, and a detonation chamber over the top plate having a bottom vent port communicating with the hole in the annular top plate. The port is closed when the top and bottom plates are engaged and the port is opened when the plates are disengaged. Rubber springs, mounted on bolts which are circumferentially disposed near the edges of the plates, resiliently urge the plates together. Detonation of a combustible gaseous mixture in the detonation chamber compresses the rubber springs and causes the bottom plate to move downwardly against the earth to create a seismic pulse. The top and bottom plates are separated and the port is opened to vent the exhaust gas. After the gas is vented, the compressed rubber springs force the plates to move towards each other, thereby closing the vent port. As disclosed in the Zens patent, the function of the rubber springs is to bring the plates together to close the vent port and to reduce chatter as the top and bottom plates meet so that a cleaner downward pulse is produced, and not to reduce the flexing of components in a recoil reducing system of a gas gun.
Loper in U.S. Pat. Nos. 3,371,740 (1968) and 3,397,755 (1968) discloses a braking system employed in a gas gun. Pressurized air is fed into and contained by a chamber with a vent port at its lower end. When the vent port opens, pressurized air is released into the water and a seismic pulse is created therein. The opening and closing of the vent port is controlled by a valve which moves downward when the valve is activated to open the vent port. As the valve moves downward, it enters a container containing water, and the water therein acts to brake the movement of the valve. Thus, Loper proposes to use confined water to brake the movement of the valve. Since the energy absorbed by water upon impact is usually negligible it will transmit the impact of the valve to the container without noticeable loss. If confined water is used to brake the impact of impinging gas upon the reaction plate in a recoil reducing system of a gas gun, the reduction of the impact will be negligible. Therefore, confined water cannot successfully be used to reduce the flexing of components in a recoil reducing system of a gas gun.